The progress of an interface supporting hot plugging such as USB allows a wide use of an external storage device such as flash memory or a removable hard disk drive connectable to a computer (host computer). The external storage device is described in JP-A-2007-4251 and JP-A-2007-141083, for example.
When one host computer is shared by two or more users, the storage area of an external storage device connected to the host computer is shared by the users. In such a case, there arises a need to prohibit another user to access the file or directory for each user stored in the storage device. In response thereto, an access control method has been proposed, which includes allocating a storage area to each user and setting the access right to a directory and a file in the allocated storage area.
However, according to the access control method, all user's storage areas can be accessed from an operating system (OS) operating in the host computer. Therefore, a malicious user may access the storage area of another user through the OS, and may tamper with, tap of a file, a directory, etc.